1. Field of the Invention
The invention relates in general to non-destructive testing of pipe, and in particular, to a test unit that is conveyed internally through pipe for magnetically inspecting the pipe for cracks or stresses and methods for inspecting associated therewith.
2. Description of the Related Art
Non-destructive testing of pipe has been done for many years utilizing magnetic particles, ultrasonic transducers, eddy current measurements, x-ray and other techniques. Operators using magnetic particle inspection techniques, typically with aid of an imaging device, can determine the existence of cracks or stresses and metal structures such as in riser pipe. Magnetic particle inspection techniques can include applying a magnetic particle medium to the surface of an area of a pipe to be inspected. The magnetic particle medium includes magnetic particles and can include other material such as florescent-type material, typically used for the inspection when conducted in a low ambient light environment. After applying the magnetic particle medium, electric power is supplied to a cable positioned to surround the area under test in order to form a magnetic field adjacent thereof. Stresses and cracks in the pipe or pipe weld produce gaps in the magnetic field into which the magnetic particles within the magnetic particle medium tend to gravitate. Visual inspection by a user either directly or through use of an imaging apparatus such as a video camera can detect the concentrations of magnetic particle medium in the vicinity of the cracks or stresses. In a low light environment, the magnetic particle medium can include means to help the user detect the magnetic particles such as a florescent means.
One type of pipe that requires such periodic inspection is a drilling riser. Drilling risers, which are utilized for offshore drilling, extend from the drilling rig to a blowout preventer and lower marine riser package, which connect to a subsea wellhead. Drilling risers are made up of sections bolted together with flanges, each section being typically from 60–90 feet in length. Each drilling riser section has a central riser pipe that is normally about 18–24 inches in diameter. Several auxiliary lines are mounted to the exterior of the central riser pipe, the auxiliary lines being used for a choke, kill and hydraulic boost purposes. The auxiliary lines are smaller in diameter and mounted parallel and offset to the axis of the central riser pipe. Normally there will be at least one weld within each riser section, this being a center weld that connects two tubular pipes together to form the riser section. Also, normally the flange connectors are mounted to the ends of the riser sections by welding. Many risers also have buoyant jackets mounted to the exterior.
A drilling vessel may have several thousand feet of riser pipe, depending on the depth to which it is rated. During use, drill pipe with drill bits on the end, casing, and other well tools are lowered through the riser. Drilling mud returns up the riser. The auxiliary lines are pressurized for various purposes from time to time. The drilling riser is re-used after each well. Consequently it is necessary to periodically inspect the riser to make sure that the welds have no weaknesses, that could result in riser pipe failure.
Inspection in the past has been accomplished by inspecting the riser pipe both deployed in a subsea environment and inspected at a land facility after retrieval. When inspected while in a subsea environment, either divers or a remote operations vehicle are deployed to perform a visual inspection along the entire length of the riser. For example, the divers can take a length of wire connected to an electrical power supply and wind it around the inspection area. The divers then spray the magnetic particle medium on the area to be inspected and provide power to the length of wire to provide the magnetic field. The divers can then directly visually inspect the area or can use a video camera, either of which may or may not be accomplished with or without additional illumination. This process can be very tedious and can be affected by obstructions such as the buoyant jackets, if installed, and can subject the divers to extreme environmental conditions.
Inspection at a land facility, on the other hand, is typically accomplished by transporting the riser sections to a facility on land that performs the inspection services. The facility removes the buoyancy jackets and auxiliary lines from each section. The riser sections are cleaned and inspected from the exterior using similar magnetic particle imaging techniques as that described. If the riser is coated with an epoxy, it must be removed at each inspection site. After inspection, the riser sections are reassembled and shipped back to the drilling vessel.
The transport of the riser sections to a testing facility on land is expensive. Also, it is time consuming to transport, clean, disassemble, inspect and reassemble the riser sections. During this time, unless a spare drilling riser can be obtained, the drilling rig would not be able to operate. Drilling rigs are very costly on a daily basis.
It has been proposed to inspect the drilling risers at the drilling vessel. Many drilling vessels have the ability to stack the riser sections horizontally on the vessel while not in use. However, there are a number of problems in doing so. The interior of the drilling riser is often not very clean, and may be coated with dried drilling mud. The central riser pipe is often out of round portions. The welded areas of the central pipe may be misaligned slightly. Also, there is normally not much access room on the drilling rig at the ends of each riser section for staging the equipment necessary to do the inspection.